Ever had a streetlight go out, or come on, just as you walk under it? Most people have, and yet it's so consistently seen as a strange and selective occurrence that it has earned its own name (Street Light Interference Syndrome), a group of people who claim to do it on cue, and an actual physical explanation.

Earlier this week we talked about the Pauli Effect, named for the famous theoretical physicist Wolfgang Pauli, who seemed to break experimental equipment just by approaching a lab. A commenter mentioned the weird phenomenon of streetlights seeming to turn off whenever they went by. This is a common enough phenomenon that almost everyone has had it happen to them at one time or another. Some people claim not only that it happens to them more often, but that it is triggered by them, or their emotions. These people even have a name, SLIders, for Street Light Interference syndrome.

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There have been few studies done on SLIders, and none have returned any measurable results. Most likely, people who notice it often are just a combination of people who take long walks at night, and people who pay enough attention to notice that it's happening. There is one thing that might cause certain people to claim SLIder status - living in a place that doesn't regularly replace its sodium vapor lamp bulbs. This leads to a phenomenon called cycling.

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Sodium vapor lamps work pretty simply. They're a glass tube filled with a gas, and sprinkled with sodium and sometimes with a few other materials like mercury. Electrical current passes through the gas, which heats up and vaporizes the other materials. The gas ionizes, and some of the electrons knock into the sodium, heating it up so it it glows. Sodium is an element that reacts readily with many different materials, and it can grab hold of the aluminum oxide that is used in the material for the lamp. This extra oxygen forms more gas inside the tube, and the pressure builds up, especially as the lamp gets hotter. As the pressure gets higher, more voltage is required to keep the electrical current going, and eventually the streetlight kicks out. When it cools down sufficiently, the gas condenses and the voltage is sufficient once more. The streetlight turns on, only to turn off again when it heats too much. So if you live in an area where the lamps bulbs are not kept fresh, you are more likely to be a SLIder.